The present invention relates generally to the field of industrial and utility boilers and furnaces and in particular to a new and useful channelized inlet for a selective catalytic reduction (SCR) system for control of NOx emission leaving the system.
SCR systems are used to clean impurities from the exhaust gases of boiler and furnaces, and in particular, to reduce NOx emissions. Ammonia is injected into the boiler exhaust gas stream in the presence of a catalyst. A chemical reaction occurs with the exhaust gas, which removes a large portion of NOx from the exhaust gas and converts it to water and elemental nitrogen.
As the catalysts used in SCR systems are carefully engineered and expensive, it is beneficial to be able to control the stoichiometry of the exhaust gas/ammonia/catalyst reaction. Ammonia is typically introduced using sparger tubes to spray ammonia into the exhaust gas stream. The sparger tubes form an ammonia injection grid (AIG).
SCR performance can be significantly affected by non-uniform flue gas flow into the catalyst, and catalyst manufacturers thus typically specify a maximum allowable non-uniformity of the flue gas velocity at the inlet of the SCR. The complex geometry of the flue gas duct upstream of the SCR, the presence of the ammonia injection grid, limited space in retro-fit applications and other factors all contribute to non-uniform gas flow within the duct. Flow correction devices such as perforated plates, splitter plates, turning vanes, hollow tube bundles, rectifier grids and other flow straighteners have all been used to produce a more uniform flue gas velocity profile and straighten the flue gas flow, thereby conditioning the flue gas flow pattern.
SCR systems are currently designed and operated to provide the highest level of flow, temperature and chemical uniformity. Techniques such as zonal AIG injection, static mixing, and judicious use of flow correction devices to provide flow conditioning, have all been used in known SCR designs to attempt to provide the most uniform flow and chemical composition practical. The goal of these designs is to produce a uniform flue gas velocity and a uniform ammonia-to-NOx (NH3/NOx) mole ratio at the inlet to the SCR. A uniform NH3/NOx mole ratio at the SCR inlet promotes efficient use of both ammonia and catalyst, while minimizing unreacted ammonia. Unreacted ammonia, referred to as ammonia slip, is undesirable as it can result in air heater fouling or baghouse blinding, and may effect the disposal or marketability of fly ash or scrubber byproducts. Maldistribution of the NH3/NOx profile at the SCR inlet is usually the major cause of high ammonia slip.
Generally, the NH3/NOx mole ratio of the NH3 needed to remove NOx is represented by the following equation:Md=η+SNH3/NOx, inlet
where Md=design mole ratio
η=NOx removal efficiency
SNH3=ammonia slip, in ppm
NOx, inlet=inlet NOx, in ppm
Thus at a low removal efficiency, e.g. η=60%, an NH3/NOx imbalance is not a significant problem. However, so-called “high performance” SCR systems are operated much closer to theoretical stoichiometry, to achieve 90%+ removal efficiency, and are thus much less tolerant of NH3/NOx mole ratio maldistribution.
Known techniques still have not eliminated the problems of NH3/NOx mole ratio maldistribution, particularly for retrofit applications where space and/or pressure drop constraints may limit design options, and for the more demanding requirements of high removal efficiency. Thus improvements that would more precisely control the NH3/NOx ratio at the inlet of an SCR would be welcomed by industry.